US President Barack Obama is expected to throw his full support behind the campaign to keep the UK in the European Union, as he arrives in Britain for a visit.

The White House has indicated that Mr Obama will set out his support for the 'remain' campaign, when he and David Cameron hold a joint press conference on Friday.

Leave campaigners, such as London's Mayor Boris Johnson, have warned Mr Obama to stay out of the debate but Deputy National Security Adviser, Ben Rhodes, told the media that the President would set out his support for the EU if he was asked.

He said: "He will make very clear that this is a decision for the people of the United Kingdom to make, it's not a decision for us to make.

"But we have no closer friend in the world, and if we are asked our view as a friend, we will offer it."

That would mean "being very straightforward and candid as a friend as to why the United States believes that it is good for the United Kingdom to remain in the European Union".

During the visit, Mr Obama will also have lunch with the Queen on Friday and hold a "town hall" style meeting with young Londoners on Saturday.